1. Field of the Invention
The present invention relates to a lithographic developer (developing solution) and a lithographic process. More particularly, it relates to a lithographic developer and a lithographic process, suited for ultra large scale integration processing.
2. Related Background Art
In recent years, as semiconductor devices are made more highly dense and fine, it has been required in lithography to form fine resist patterns having a processing latitude. Hence, photoresists having a high resolution are used to form the patterns. Such resists are of a positive type or a negative type. In usual instances, the patterns are formed using an alkaline developer as exemplified by a 2.38% by weight aqueous solution of tetramethylammonium hydroxide (hereinafter "TMAH").
As resist materials have been made to have a higher resolving power, the ratio of dissolving rate at exposed areas of a resist layer to that at unexposed areas thereof, i.e., the selection ratio of a resist to a developer has been improved. The state thereof is shown in FIG. 16. In FIG. 16, the ratios of dissolving rate at exposed areas of a resist layer to that at unexposed areas thereof are shown as functions of TMAH concentration, in resists having different resolutions. In the drawing, a positive photoresist A has a resolution of 1.2 .mu.m, a positive photoresist B, 0.8 .mu.m, and a positive photoresist C, 0.6 .mu.m. As shown in the drawing, the selection ratio a resist has is seen to exponential-functionally increases with an improvement in the resolution of the resist. In other words, an achievement of a resist having a high resolution is seen to be equal to an improvement in the selection ratio at exposed areas of a resist layer and unexposed areas thereof.
In order to improve developability, it is attempted to increase the concentration of a developer. Such an attempt, however, leads to a decrease in the selection ratio at exposed areas of a resist layer and unexposed areas thereof, resulting in a deterioration of the-resolution performance of resists.
FIG. 17 shows a 0.55 .mu.m line-and-space resist pattern developed by a developer of a conventional type in which no surfactant is added. Exposure was carried out using a g-line reduction aligner having a lens with a numerical aperture of 0.43. Because of a poor wettability of the developer on the resist surface, trailing and residues (shown by the encircled portions, 1201, in FIG. 17) of the resist pattern are seen to have occurred after development. In order to prevent such a phenomenon, a developer containing a surfactant has been used in some instances. When, however, some types of substrates are used, none of conventional developers containing a surfactant can prevent the resist residues from occurring. Conversely, the addition of some types of surfactants has caused a decrease in the sensitivity of resists or a decrease in exposure latitude.
FIG. 18 shows the flatness of the surface of a silicon wafer having been immersed for 60 seconds in the developer of a conventional type in which no surfactant is added, which flatness is indicated as the central-line average roughness. As is seen therefrom, the wafer having been immersed in the developer has a very rough surface only because of its contact with the developer for a time as short as 60 seconds, compared with a reference wafer having not been immersed in the developer. Fabrication of semiconductor devices in such a state of surface brings about an extreme deterioration of electrical characteristics. FIG. 19 shows data obtained by examining a channel mobility of MOS transistors fabricated by the use of wafers having a rough surface. The channel mobility is seen to be extremely poor with respect to the reference value when the surface has roughed upon its contact with the developer.
The silicon surface can be prevented from roughing when a certain surfactant is added to the developer, but the surfactant is adsorbed onto the silicon. FIG. 20 shows data obtained by observing the carbon 1s peak by X-ray photoelectron spectroscopy (XPS). A peak 1501 that shows the presence of the surfactant was seen in the data from the silicon surface exposed for 60 seconds to the developer containing a surfactant. It was found that, when the developer containing a surfactant was used, the surfactant was adsorbed to the underlying substrate and was not removable by usual rinsing. Adsorption of the surfactant to the substrate surface caused a carbon contamination to bring about a lowering of film quality in the subsequent formation of various kinds of thin films. On the other hand, when a surfactant not adsorptive to the silicon surface was selected, the surface of silicon roughed.
Under existing circumstances, no resist can show its ability when contact holes are formed using a resist having a resolution up to 0.6 .mu.m (capable of forming-line-and-space patterns with a mask fidelity).
FIGS. 21A to 21F show cross-sectional photographs of contact holes developed using a conventional developer. In the case of exposure conditions under which contact holes with a size of 0.8 .mu.m are formed (220 mJ/cm.sup.2), 0.7 .mu.m and 0.6 .mu.m holes are in a condition of under-exposure.
In the case of exposure conditions under which contact holes with a size of 0.7 .mu.m are formed (280 mJ/cm.sup.2), 0.8 .mu.m holes are in a condition of over-exposure and 0.6 .mu.m holes are in a condition of under-exposure. Namely, in regions having a smaller exposure area in themselves (e.g., hole regions), the developer had so poor a wettability that it did not penetrate into the resist pattern and hence no satisfactory resist performance was exhibited.
In order to improve the wettability of developers, developers containing a surfactant are used. FIG. 22 shows the relationship between the concentration of a surfactant added to a developer and the exposure threshold energy for the formation of contact holes. The contact hole exposure threshold energy is meant to be an exposure energy necessary for the bottom of a contact hole to reach the underlying substrate. The exposure threshold energy decreases with an increase in the concentration of the surfactant added to the developer, but the difference in exposure threshold energy between different hole sizes does not change from the state before the addition of the surfactant. Namely, when conventional surfactants are used, it Gas been impossible to form fine contact holes with different size, in a high precision under the same exposure conditions.
FIGS. 23A-C show cross-sectional photographs of contact holes formed using developers containing the surfactant as shown in FIG. 22. FIGS. 23A-C show that when the conventional surfactants are added the side walls of contact holes have a curved shape. When the amount of a conventional surfactant added was increased to improve the wettability of the developer, a deterioration of hole configuration occurred.